1. Field of the Invention
The present invention relates to an optical information reproduction apparatus and an optical information reproduction method, and it is suitable for use in, for example, an optical information recording and reproduction apparatus for recording and reproducing information on an optical disc having plural information recording layers.
2. Description of the Related Art
In recent years, in optical information recording and reproduction apparatuses for recording and reproducing information on a recording medium such as an optical disc, a technique for making recording information into a higher density by a shortening of the wavelength of a light source and by a higher aperture number of an objective lens has been proposed. For example, a two-layer disc having a recording capacity exceeding 23 Gbytes per layer as a result of using an objective lens with the aperture number 0.85 and a bluish purple semiconductor laser of a wavelength 405 [nm] has been standardized as a “Blu-ray Disc (trademark)”. Furthermore, in recent years, as the density of recording information becomes higher, a lot has been reported about multilayer optical discs for optically reading, from one side, recording information layers that are formed in plural layers.
When the recording information of a particular information recording layer Ln within such a multilayer optical disc is reproduced, if the distance between adjacent layers is sufficiently large, influences due to the leakage of signals (hereinafter, referred to as “interlayer crosstalk”) from a plurality of information recording layers other than the information recording layer Ln are small, and the reproduction characteristics of a RF signal does not particularly pose a problem.
The smaller the interlayer distance, the larger the interlayer crosstalk, and the RF signal is deteriorated. The smaller interlayer distance is caused by a limitation of the distance from the disc surface to each information recording layer due to the number of layers of a multilayer optical disc, the correction range of an expander for correcting spherical aberration caused by the difference in the distance from the disc surface to each information recording layer by a higher aperture number of an objective lens, an inclination margin between the objective lens and the disc, and defects such as scratches and fingerprints.
As a technique for reducing such interlayer crosstalk, there is a known pickup device in which the standardized detector size of an optical detector is optimized (refer to, for example, Japanese Unexamined Patent Application Publication No. 2002-25098).